The plum curculio (PC), Conotrachelus nenuphar (Herbst) (Coleoptera: Curculionidae), is a weevil insect that directly attacks the fruit of a wide range of stone and pome fruits. It is native to North America, east of the Rocky Mountains, and presumably fed on native wild Prunus and rosaceous hosts before the availability of cultivated fruits. In the eastern U.S., PC is an annual threat to commercial orchards, causing significant economic loss if not controlled. PC is controlled in commercial orchards primarily with standard insecticide cover sprays beginning at petal fall. PC was not known in the West until a small, isolated population was detected in the Brigham City area of Box Elder County, northern Utah in the early 1980’s. It was first identified in Utah from sweet cherry and plum. In Utah and the West, PC is a quarantine insect and can prohibit or reduce the export of fruit to certain states and countries.

Where is PC in Utah and What Does it Eat?

Surveys for plum curculio injury to fruit or adults caught in traps were conducted in 1998 and 1999 in Box Elder County. Survey sites were concentrated along Highways 38 and 89 from Honeyville in the north, through Brigham City residential areas, and to Willard in the south. PC oviposition scars on fruit and eggs and/or larvae in fruit were found in sweet cherry, tart cherry, wild plum (Prunus americana), cultivated plum, apple, and apricot. No PC injury was found in peach or pear. Peach is a likely host, but evidently not a common one in Box Elder Co. Pear is likely a poor or non-host. Sweet cherry and wild plum are the most common fruit hosts in Box Elder Co. PC has primarily been found in neglected sites that do not receive insecticide sprays, pruning, etc. These sites are home yards (predominantly sweet cherry), roadside wild plum, and neglected orchards (predominantly tart cherry). Based on a survey of over 300 sites in Box Elder Co., 99 host tree sites (approx. 1/3 of the total number of sites surveyed) were infested with PC. The majority of infested sites are in residential areas of Brigham City (73 sites) and most are sweet cherries in home yards (50 sites). The phenology of cherry and plum seems to be most closely aligned with that of over- wintered adults emerging in the spring and seeking green fruit for feeding and oviposition.

PC Biology – How is PC Making its Living?

The Utah population is univoltine (1 generation per year), like the northern strain in the East, with an obligate (i.e., mandatory) diapause during the adult stage from approximately late September through April. Over-wintered adults begin to emerge in late April with peak emergence coinciding with late petal fall to early fruit set in cherry and plum (Figs. 1A & 1B).Adults are in host trees, ready to feed and oviposit before green fruit is available. The majority of eggs in sweet cherry occur from late May to late June, while peak larva densities are found from June to mid-July (Fig. 1A). The phenology of PC eggs and larvae in fruit overlaps substantially with the fruiting period of plum, cherry, peach, and apple, but not with apricot (phenology of apricot is earlier than PC). A second, smaller peak of adults occurs during August and September (Figs. 1A & 1B), which is the summer generation adults feeding in host trees before moving to overwintering sites.

The sex ratio of adult PC caught in traps is skewed toward males during the majority of the season at most sites (female proportion <0.5) (Fig. 2). A greater proportion of males were caught in traps especially in the spring and late in the summer, which means that males are the first to arrive in fruit trees and the last to depart. These results could also indicate that traps preferentially catch males over females. A high proportion of female PC collected in the spring were already mated with mature eggs (Fig. 3). Summer generation females entered diapause unmated and with immature ovaries. Therefore, mating appears to occur during the early spring as PC emerges from overwintering sites or shortly after dispersal to host trees. If males do arrive first in fruit trees, and they could be eliminated or prevented from mating with females, then early-season control methods may be effective. Ideally for the Utah situation, the development of effective chemical and non-chemical control options appropriate for non-commercial tree fruits are needed.

Important Findings on PC Biology Relevant to its Control

Findings on PC biology and life history emphasize the importance of timely control of the insect in the spring, before eggs are laid in the young, green fruit. Adults emerge from overwintering sites, seek out suitable host trees, and mate before host fruit is available. Therefore, insecticide treatments applied after petal-fall in sites with abundant PC will not prevent early oviposition and feeding injury. Insecticides applied before petal-fall may miss later emerging adults as peak emergence goes on until mid-June, and so additional application(s) would be necessary. Based on research conducted in the East, petal-fall is the optimum timing to protect fruit from PC injury and prevent development of the summer generation in fruit. A second application may be necessary to extend protection if PC densities are high and still active in host trees while fruit is susceptible. Elimination of blossoms using a bloom-eliminator product and removal of trees are other tactics to reduce PC populations in infested areas.

Adult PC Monitoring – How to Detect PC Adults Where They Occur in Utah?

Adult trapping methods have been evaluated to find an effective method to monitor PC. Because PC is a quarantine pest, agriculture export regulations require monitoring in all major fruit producing areas of the state. Hand checking of fruit for injury is time consuming and must be conducted at multiple times to sample all fruit types when they are most attractive to PC. Adult PC primarily feed and lay eggs in green fruit. Infested fruit drop from trees making it difficult to detect in the late summer and fall.

In 1998, pyramid traps (Tedder’s weevil trap), twig-mimic traps (based on R. Prokopy design), sticky trunk bands, and limb jarring were evaluated for capture of PC adults in 4 orchards (tart and sweet cherry). The only type of attractant evaluated was mechanically injured green fruit. The pyramid trap was more effective (P=0.02)in attracting and capturing adult PC than all other sampling methods (Fig. 4). The addition of green fruit did not significantly enhance trap catch, although numerically catch was greater in traps baited with green fruit than without (Fig. 4).

In 1999, pyramid and circle (enlarged pecan weevil circle trap) traps were
evaluated with no-bait, PC aggregation pheromone, fruit essence (fruit odor
from sweet and tart cherry, and plum), and plum extract (fruit pulp). There
was not a significant difference in capture of PC adults in pyramid and circle
traps in 3 orchards (tart and sweet cherry) (Fig. 5). The circle trap is smaller,
easier to handle and deploy, and easier and less expensive to make than the
pyramid trap. Circle traps are attached directly to the tree trunk, whereas
pyramid traps are placed on the ground adjacent to the trunk. The circle trap
is better suited for home yards and for orchards where the ground cover is mowed
or the soil is cultivated near the tree rows.

Some of the attractants added to pyramid and circle traps in 1999 did enhance
trap catch, but the results were variable among experimental sites (Figs. 6-8).
The small size of many sites (i.e., low number of trees), low number of replications
within sites, and strong clustering of PC in sites not related to trapping treatments,
all created difficulties in detecting significant differences between attractants.
In four of the five cherry sites, cherry fruit essence (sweet and tart) enhanced
trap catch over unbaited traps (Figs. 6 & 7). Trap catch with the pheromone
was variable, and it is likely that the pheromone dispenser was not releasing
the correct chemical blend after a short period of time. The plum extract seems
to have performed well in the wild plum-trapping site (Fig. 8), increasing adult
catch up to 5 times over other attractants and unbaited traps.

How Should We Manage PC in Utah?

In conclusion, what is the bottom line for dealing with this quarantine insect in Utah? We need to fully determine where it occurs and what it’s eating. Surveys to detect PC in other areas of Box Elder County will continue in 2000. If possible infestations of PC are detected in any other counties; these sites will be investigated. Also the ability of PC to feed and oviposit in a broad range of commercial and wild hosts (e.g., wild plum, black hawthorn, serviceberry, chokecherry) will be determined. We need to know all the possible fruit hosts for PC in Utah. Finally, it is going to require a community-wide effort to control and reduce PC. Because the insect is established in home yards, roadside sites and neglected orchards, public education should play a major role in the management of PC. The public needs to be educated that neglected trees can be a source of this insect, and removal of such trees should be encouraged. The Utah fruit industry and agriculture regulatory and education organizations are already involved in the management of PC. The next steps involve working more closely with the communities where PC occurs to prevent its spread and reduce its incidence.